Systems in which an output by a speaker enters a microphone, such as a hands-free calling system and a telephone conference system, need to ensure their call quality, particularly to cancel echoes caused by acoustic coupling occurring on a two-way call. For a technique of cancelling the echoes, there is an acoustic echo canceller that has an adaptive filter which identifies a transfer function between a speaker and a microphone, generates a pseudo echo signal using an echo path identified by the adaptive filter, subtracts the pseudo echo signal from an input signal of the microphone, and thereby cancels an echo transmitted from the speaker to the microphone. A conventional acoustic echo canceller cancels echoes by means of a single adaptive filter. However, since an acoustic signal has an unbalanced energy distribution in a frequency band, there is a drawback that sufficient echo suppression cannot be performed with the single adaptive filter due to a convergence delay, etc.
As an acoustic echo canceller that solves such a drawback, there is a sub-band echo canceller which splits an acoustic signal into a plurality of frequency bands and performs echo cancellation on an individual band basis. In the sub-band echo canceller, since learning by an adaptive filter is performed on a narrow bandwidth basis, elimination of the influence of an unbalanced energy distribution can be expected. In addition, since the sampling frequency can be reduced by splitting the band, a reduction in the amount of computation can be expected. More specifically, assuming that the band (0 to 4 kHz) of a signal having a sampling frequency of 8 kHz is split into two sub-bands of 0-2 kHz and 2-4 kHz, a bandwidth of each sub-band is 2 kHz. Since the signal of each sub-band can be represented as a signal with a sampling frequency of 4 kHz, the sampling frequency can be reduced to half.
However, there is a problem, due to the influence of aliasing during downsampling, that the echo existing near a boundary between the split bands is not sufficiently eliminated, causing a residual echo. FIG. 5 shows an illustrative diagram depicting the aliasing caused by downsampling. When a signal with a sampling frequency of 16 kHz is downsampled to convert the sampling frequency to 8 kHz, if there is a signal component in a frequency band higher than one-half of the sampling frequency, then a folding signal, i.e., aliasing, is generated.
In Patent Literature 1, after separating an input signal of a microphone into a high-band component and a low-band component through a quadrature mirror filter (QMF), a thinning process on each separated component is performed for downsampling. Therefore, echo canceller for each band can be achieved with a small amount of computation. In addition, by providing a high pass filter (HPF), which allows only high-frequency components to pass therethrough, for the low-band component of the input signal, an echo signal existing near the band boundary can be eliminated.